(aka phono jack) The Radio Corporation of America (RCA) originally developed this type of pin connector for internal chassis connections in radios and televisions during the '30s. It became popular for use in the cables that connected phonograph cartridges to preamplifiers because it was inexpensive and easily fitted to the rather small diameter shielded cables used for the cartridge leads (then they were mono cartridges so single conductor shielded cables were adequate). Now the standard for use in line level consumer audio and project studio sound equipment, and most recently to interconnect composite video signals.